Recent years have witnessed a rapid spread of wider bandwidth technologies in communications infrastructure as well as dramatic advances in the information-processing ability of computers. In accordance with such advances, information processing circuit substrates having high-speed information transmission paths have been required than ever before. In this context, signal transmission using light is being studied as a means for overcoming the transmission speed limits of electric signals. Polymer optical waveguides, which can be processed inexpensively, have received attention as circuit substrates that can transmit signals optically.
Known methods for manufacturing polymer optical waveguides involve, for instance, lithographic patterning of a layer comprising a photosensitive polymer (for instance, Patent document 1 or Non-patent document 1). An outline of such a method is set forth below.
Firstly, a film layer comprising a photosensitive polymer for forming a core is formed on the surface of a lower cladding layer that is formed on the surface of a substrate. The photosensitive polymer film layer thus formed is then irradiated with UV rays, via a photomask having formed thereon a circuit pattern, to selectively cure thereby the portions where the core is to be formed. The core is formed then by developing, whereby uncured portions are removed. The core is covered with a material for forming an upper cladding layer.
The film layer for core formation may be formed, for instance, in accordance with the following method. A liquid photosensitive polymer or a varnish of a solid photosensitive polymer is coated onto the surface of a lower cladding layer, formed on the surface of a substrate, and is then dried. In another method, a photosensitive polymer varnish is coated first onto the surface of a support substrate, and is then dried to form a dry film. This dry film is then laminated onto the surface of a lower cladding layer that is formed on the surface of a substrate.
Patent document 1: Japanese Patent Application Laid-open No. 2007-084765
Non-Patent document 1: “Optical-Electrical Flexible Printed Wiring Board”, MEW Technical Report, Vol. 54, No. 3 (September 2006)